The reinforcement of inorganic binders such as cement and concrete with glass fibers resistant to alkaline compounds is well known in the prior art. However, when these glass fibers are incorporated into binders of the type described above, a problem arises with regard to the strength and durability of the resultant binder-fiber composite. The composite product thus produced deteriorates over time, leading to a reduction in the breaking stress, i.e., the moduli of rupture, which seriously affects the durability of the product. The deterioration of the product is presaged by a sudden drop in the elongation in time, as well as in the energy required to rupture the fibers.
The degradation mechanism comprises a chemical attack upon the fibers by the alkaline interstitial solution within the cement, coupled with a physical manifestation wherein the cement-fiber interface is modified by the growth of crystals of Ca(OH).sub.2 and of hydrated silicates. This latter phenomenon dominates in the case of glasses which are resistant to alkalines. Because of the negative impact of such an attack on the durability of the product, the use of cement reinforced with glass fibers of the above-defined type is confined to nonstructural applications in the construction industry. In addition, considerable precautions are taken when such cement based products are utilized, e.g., by the use of flexible setting techniques and an attempt is made to take up the dimensional variations.
To eliminate these problems, more efficient glass compositions have been sought. Ca(OH).sub.2 is the principal enemy of glass fibers. In this regard, French Pat. No. 2,149,998 describes a product containing cement and glass fibers which are resistant to alkalines having certain defined characteristics. This product can attain a modulus of rupture of 24.5 MPa after a month under 60.degree. C. hot water. To attain these values, a predetermined amount of a "pozzolan" is added to the cement matrix, the term pozzolan being defined in the patent as encompassing both artificial and natural materials which contain vitreous silicates that can react with lime.
However, this solution to the problem of glass degradation exhibits the following drawbacks:
the reference does not specify any composition for the pozzolans to be used. The experience of the applicant and the relevant literature both show that all pozzolans will not lead to an improvement in the strength of cements reinforced with alkali-resistant glass.
the reference discloses examples where the moduli of rupture and the impact strength of the composite, despite the addition of a pozzolan, diminish considerably over time.
For example, the modulus of rupture in the case of a mortar consisting of ordinary portland cement and 25% pozzolan of the fly ash type exceeds 29.46 MPa after 7 days but becomes lowered to 16.77 MPa after 30 days, including 23 days under water at 60.degree. C.